This invention relates to diode lasers and more particularly, a configuration of and a method for optical beam shaping of diode laser bars to produce an optical beam of high power and brightness, allowing for efficient coupling of the optical diode laser bar output into an optical fiber.
High power solid state lasers and particularly high power fiber lasers generally depend on the availability of optical pump beams with high optical power and brightness, i.e., the required pump power needs to be made available in as small a space as possible. Semiconductor diode laser arrays are firmly established as the main source of optical beams of high power, i.e., 1 watt and above including ultra-high powers of greater than 10 watts. With current semiconductor technology, a power approaching 40 kW can be produced by a stacked diode array with dimensions of only 5xc3x9710 cm (R. J. Beach et al., Laser Focus World, December 2001). The stacked diode array typically consists of individual diode bars of around 1 cm in length, which in turn incorporate 10-20 individual emitters separated by 0.2-1.0 mm. The laser beam originating from one individual emitter typically has a divergence of 10xc2x0xc3x9750xc2x0. The small divergence beam is in the plane of the x axis and the high divergence beam is in the plane of the y axis. As shown in FIG. 1, a laser diode 101 emits laser beam 102 having relatively small divergence in the direction of the x axis but high divergence in the direction of the y axis. These axes are sometimes referred to as slow and fast axis respectively. The fast axis beam is generally diffraction limited and can be collimated with a cylindrical lens aligned parallel to the slow axis of the single emitter, i.e., laser diode 101. The slow axis beam is typically far from diffraction limited.
The brightness B of the optical beam from an individual emitter can be calculated as B=power/(emitting areaxc3x97angular divergence). For an individual emitter of dimensions 1xc3x97100 xcexcm operating at a power of 2 W we obtain B=14 MW/cm2. In contrast, the brightness of a 20 element diode bar with dimensions 1 xcexcmxc3x971 cm operating at a power level of 40 W is only about 3 MW/cm2, whereas the brightness of a diode stack as described above operating at a power of 40 kW is only of the order of 5 kW/cm2. Indeed, the stringent requirements for cooling of a diode stack require that the individual diode bars and emitter areas are substantially spaced apart, greatly increasing the emitter area and limiting the brightness of such high-power laser systems.
The brightness of diode laser beams has traditionally been increased by the implementation of beam-shaping optics, i.e., by optically combining the individual emitter beams from the diode array to generate a single optical beam which can be efficiently coupled into an optical fiber (See, e.g., U.S. Pat. No. 5,168,401 of Endriz, hereinafter Endriz ""401). The brightness B of the fiber-coupled optical beam can be calculated as B≈P/(A*xcfx80NA2), where P is the coupled power, A is the core area and NA is the numerical aperture of the fiber.
Optical beam shaping is possible by a variety of means. As shown in FIG. 2, most beam-shaping methods transform the optical beams such that the beams from the individual emitters 201, 202 and 203 form a picket fence 209, 210, 211 with the fast axis beams aligned parallel to each other. Since the beams 204, 205 and 206 are diffraction limited along the fast axis (ie., in the y direction as depicted), very tight packing of the emitters along the fast axis is possible. To facilitate optical beam transformation, collimation of the fast axis beam is also implemented with a cylindrical lens 207 as shown in FIG. 2 followed by beam shaping optics 208. The generation of a properly aligned picket fence comprising collimated beams 209, 210 and 211 is key to most industrially relevant beam shaping methods. We can distinguish four different classes of picket fence generation methods and systems.
A first class of methods generates the picket fence by optical rotation of each emitter beam by 90xc2x0, where the direction of the emitter beam is further deflected by around 90xc2x0 after reflection from at least two reflecting surfaces. The Endriz ""401 patent describes such an example. Further examples of such a method are U.S. Pat. No. 5,418,880 of Lewis et al. and U.S. Pat. No. 6,044,096 of Wolak, et al.
A second class of methods generates a picket fence by optical beam rotation based on beam-rotating prisms such as the Abbe-Kxc3x6nig prism as disclosed in U.S. Pat. No. 5,243,619 of Albers, et al. The advantage of this design is that it avoids a 90xc2x0 deflection of the beam direction such that only a small displacement in the propagation direction results.
A third class of methods of generating a picket fence is based on beam deflection in a set of multi-facetted mirrors or prisms. In these methods a first multi-facetted optical structure deflects the beam to obtain some beam spacing in the y direction, while a second multi-facetted optical structure deflects the beams to overlay the beams along the x direction (see, e.g., U.S. Pat. No. 5,887,096 of Du et al. and U.S. Pat. No. 6,151,168 of Goering et al.). Such systems do not require beam rotation optics, but generally comprise a beam deflection along the propagation direction, relying on the manufacturing of expensive high precision multi-facet optics. The function of beam deflection and overlay can also be accomplished in one single optical element as disclosed by U.S. Pat. No. 5,825,551 of Neilson, et al. A limitation of the approach taken by Neilson et al. is the variation between the optical path lengths of each individual emitter beam through the beam shaping optic, which in-turn limits the focussability of the resulting beam. Note that the first three classes of beam-shaping optics use non-focussing optics.
A fourth class of methods of generating a picket fence is based on beam rotation in a transmissive optical element that comprises at least one cylindrical surface. (See, e.g., Lissotschenko et al., German Patent No. DE 19920293.) A representation of this structure is shown in FIG. 3. As shown therein, optical element 301 comprises an array of cylindrical lenses 302, 303, 304 and 305. The arrangement provides for a 90xc2x0 rotation of respective laser beans such that a laser beam depicted as rays 306a, 306b and 306c undergoes to 90xc2x0 rotation in a plane perpendicular to its direction of transmission along the Z axis as it transits cylindrical lens 303. Similarly, rotated in the x,y plane are laser beans 307a, 307b, 307c and 308a, 308b, 308c by the respective cylindrical lenses 304 and 305. Recently, the technique by Lissotschenko et al. was extended (see U.S. Patent Application Publication No. US2002/0015558) to include also beam rotation cylindrical lenses with an additional concave curvature, where the central axis of the concave surface is orthogonal to the main cylinder axis. Such a cylindrical lens was referred to as a concave toroidal surface. A limitation with these techniques is that generally cylindrical lens arrays need to be manufactured for beam shaping of a whole diode bar, which are difficult to coat with anti-reflection coating due to the presence of large curvatures and multiple crevices in the optics. Moreover, another limitation with the technique by Lissotschenko et al. is that no beam homogenization elements are used. Without beam homogenization tight packing of individual beamlets from a diode bar along the fast axis is limited and therefore a significant reduction in brightness results.
Generally all techniques described so far are not monolithic and therefore complex alignment procedures are required, leading to high manufacturing costs. Moreover, the application of optical coating is also difficult in these devices which can limit the optical throughput.
The present invention is directed to an optical arrangement allowing efficient coupling of a diode bar into an optical fiber. The brightness of the beam emitted from the diode bar is maximized by incorporating a beam inversion optic with a magnification M of M=xe2x88x921. The beam inversion optic is based on arrays of graded index (grin) optic, cylindrical Fresnel lenses, reflective focusing optics or a more general optical system. The graded index optic can be planar, circularly symmetric, circularly symmetric with arbitrarily shaped input and output surfaces and can comprise more general beam inversion designs. The reflective focusing optics can conveniently comprise cylindrical reflective mirrors. For the case of a planar graded index optic, beam inversion is obtained by aligning the lines of equal refractive index at an angle of approximately xc2x145xc2x0 (as used herein, the xe2x80x9cxc2x1xe2x80x9d sign refers to an angular displacement of the specified amount in both positive and negative rotational directions and not to a range of angular displacements) with respect to the slow axis of the individual emitters. In the case of cylindrical Fresnel lenses, the lines of equal phase retardation are equally aligned at an angle of approximately xc2x145xc2x0 with respect to the slow axis of the individual emitters. In the case of circularly symmetric graded index lenses, the lens axis is aligned at an angle of approximately xc2x145xc2x0 with respect to the slow axis of the individual emitters. Equally, more generally a shaped beam inversion optic has at least one of the optic axes aligned at an angle of approximately xc2x145xc2x0 with respect to the slow axis of the individual emitters. Beam inversion is further facilitated by first collimating the fast axis of the individual emitters with a single cylindrical lens aligned parallel to the slow axis of the diode bar. Additional collimation of the slow axis beam can also be implemented. Multi-faceted lenses can further be used for beam homogenization. Highly integrated monolithic beam inversion optical systems are also possible. The monolithic beam inversion optical system comprises an integrated fast axis collimation element in conjunction with a beam rotation element. An additional slow axis collimating element can also be incorporated in a monolithic fashion; a monolithic construction is further also possible with an additional multi-facet beam homogenizing element, collimating the fast axes of the individual beam rotated beamlets. The beam inversion optic can further be used for beam shaping of the output of individual emitters, facilitating efficient coupling of the output into optical fibers. Ultra-high power optical beams can be obtained by implementing the beam-inverting optic with stacks of diode bars. Alternatively, ultra-high power optical beams can be obtained by combining the output of individual fiber-coupled diode bars into an optical fiber bundle that is operated in conjunction with an efficient heat sink.